ABSTRACT
The basic chain structure of a silicone sealant consists of alternating silicon and oxygen atoms with organic groups, primarily methyl on the silicon atoms. Typi cally acetoxy groups are present at the end of the polymer chains for further po lymerization and cross-linking into an elastomer network, producing acetic acid as a by-product of the moisture cure reaction. Alkoxy and oxime types, which, re spectively, evolve alcohols and methylethylketoxime (MEKO) by-products through moisture cure, are also more and more prevalent in the market. These lat ter systems are preferred for metal surfaces, concrete and electrical circuit devices because of corrosion, which occurs when using an acetic acid curing system. A typical silicone sealant formulation consists of the following ingredients: about 60% by weight of high viscosity (20-1 10x103 mPa.s) hydroxy-terminated polydimethysiloxane (PDMS) polymer, 10-30% by weight of lower viscosity (100-1000 mPa.s) trimethylsilyl-terminated PDMS fluid, 4-11% by weight of a reinforcing filler - fumed silica being the most commonly used, 3-5% by weight of cross-linker, and 0.05-2.0% by weight of a catalyst - the amount of the last in gredient depends on which curing system is used (acetic, oxime or alkoxy). Often an adhesion promoter system is used to enhance adhesion strength between build ing components, for instance. These adhesion promoters are typically organofunctional silanes, also known as coupling agents [2-4]. Some silicone sealants are also extended using either non-reinforcing fillers like ground calcium carbon ate, which is cheap and readily available from various commercial sources, or liq uid organic extenders that maintain the transparency of silica filled systems.
